Conventionally, various types of sensors which detect push amounts are devised as techniques used for a touch input device which can detect a push amount. For example, Patent Literature 1 discloses a sensor which includes a pressure sensing unit and a sensing result converting unit.
The pressure sensing unit includes a piezoelectric body of a flat shape, and outputs a signal of an amplitude level corresponding to a displacement amount of a piezoelectric body. The sensing result converting unit integrates output signals of the pressure sensing unit. This integration arithmetic operation value corresponds to the push amount. Thus, the sensor of Patent Literature 1 senses the push amount.
PTL 1: Japanese Patent Application Laid-Open No. 2000-283869
However, in the piezoelectric body, a signal level of a displacement amount obtained when a pressing force is applied, and a signal level of a displacement amount obtained when the pressing force is released are different. Hence, when signal levels are integrated as in the sensor according to Patent Literature 1, a reference potential differs before and after one pushing operation. Hence, according to the configuration of the sensor according to Patent Literature 1, the reference potential differs per pushing operation.
In this regard, the push amount is calculated from an integration value of differences between signals levels corresponding to displacement amounts, and the reference potential. Therefore, when the reference potential changes, a relationship between the signal level and the push amount changes. That is, even when the push amount is the same every time, the difference between a signal level and the reference potential differs. Hence, according to the configuration of the sensor according to Patent Literature 1, precision to measure the push amount lowers.
Particularly when the pushing operation is repeated, the relationship between the reference potential and the pressing force (push amount) loses touch with each other. Therefore, the measurement precision further lowers.